A primary goal in powder metallurgy is to achieve high density of compacted and sintered bodies. There are several methods of improving density, one of those methods is warm compaction which improves the compressibility of the powder giving a green body with higher green density. By applying die wall lubrication, which makes it possible to minimise the amount of internal lubricants used, the green density may also be increased. The use of high compaction pressures in combination with low amounts of lubricants also results in elevated green densities. Soft annealing of a stainless steel powder, where the material is strain relieved and recrystallized, also improves the compressibility. After compaction the green body is subjected to a sintering operation in order to achieve a sintered body. High temperatures at sintering, i.e. above about 1180-1200° C. lead to increased shrinkage during sintering and higher density of the body. However, high temperature sintering requires specially equipped sintering furnaces. Additionally the energy consumption will be increased.
Special problems are encountered when high density, stainless steel PM parts are manufactured due to the presence of chromium, which makes the steel resistant to corrosion.
Stainless steels have approximately above 10% chromium. Most often carbon is present in steels and will cause formation of chromium carbides. The formation of chromium carbides lowers the chromium content in the matrix, which in turn causes lower corrosion resistance. In order to avoid that the chromium content in the matrix is reduced, carbide forming stabilizers, such as niobium, are often used. In this way the formation of chromium carbides can be avoided and instead niobium carbides are formed, a result of which is that the corrosion resistance can be maintained. However, a problem with the use of niobium is that high sintering temperatures are necessary for obtaining high sintered densities and the energy consumption is considerable.
It has now been found that, by using the new powder according to the present invention, the energy costs for producing sintered stainless steel PM parts can be reduced. Another significant advantage of using the new powder is that a comparatively higher sintered density can be obtained.
The sintered parts manufactured by using the new powder are of particularly interest within the automotive industry where the demands on both costs and performance of the parts are high. The new powder can also be used for sintered parts in exhaust systems, and especially for flanges in exhaust systems.
The present invention concerns stainless steel powder, stainless steel powder compositions as well as the compacted and sintered parts obtained thereof having high densities. Specifically the invention concerns stainless steel powder compositions for manufacturing powder metallurgical parts.